In a medical operation involving the percutaneous insertion of an elongated instrument to a predetermined position within a patient's body, the direction and the insertion depth as well as the entrance position at the patient's skin have to be determined. When the medical operation in question is a biopsy operation performed by means of computerized tomography (CT), the direction and the insertion depth can be calculated from coordinates given on a CT screen. Regarding the entrance position, a CT scanner provides information about where a particular picture of a slice has been taken, i.e. the position along the length of a patient's body is known, but the position transverse to the body extension is unknown. Both a transverse coordinate and a longitudinal coordinate are needed to determine a correct entry point. To facilitate the establishment of the correct entry point at the skin surface, some kind of grid can be arranged on the skin at the approximated entry area.
In its simplest form, such a grid may consist of a number of catheters or similar items that the medical personnel attach on the skin surface. A more dedicated grid has, however, previously been proposed by the assignee of the present applicant. This grid consisted essentially of a generally rectangular frame, with a number of transverse grid bars extending between the long sides of the rectangular frame. The grid was intended and designed for use together with a special CT-guidance device, and its construction regarding versatility and user-friendliness can be improved.